Emissions Scenarios - IPCC

An Overview of the Scenario Literature 91
other trends discussed in this chapter, increases are generally
modest; numerous scenarios even depict a long-term decline in
emissions. Thus, SO2 emissions are invariably projected to be
decoupled progressively from their underlying driving forces of
increases in population and economic activity, and hence
energy demand. The median across all scenarios indicates a
gradual increase of some 22% over 1990 levels by 2050, and a
return to 1990 levels by 2100. Only two scenarios exceed the
range of increases in long-term SO, emissions spanned by the
IS92 scenario series.
0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
Range (index, 1990=1)
9 12 15 18 21 24 27 30 33 36 39
Range
(GtC)
Figure 2-2b: Histogram sliowing tlie frequency distribution
of global CO2 emissions in 2100 for 190 scenarios. The first
horizontal axis shows indexed emissions (1990 = I); the
second axis indicates approximate absolute values by
multiplying the index by the 1990 value (5.9 GtC, see
footnote 4). For reference, the emissions of the IS92
scenarios are indicated. The horizontal bars indicate the
ranges for the intervention, non-intervention and nonclassified
scenario samples respectively. The frequency
distribution associated with scenarios from the literature does
not imply probability of occurrence.
2.43. Sulfur Dioxide Emissions
An overview of global long-term scenarios of SOj emissions is
shown in Figure 2-3. Altogether 81 scenarios in the scenario
database report SOj emissions. Most scenarios were published
after 1995, which indicates the importance of the influential
and innovative emissions included in the previous IFCC
scenario series IS92 (Pepper et al, 1992). Apparently, they
stimulated research on long-term frends and impacts of SOj
emissions.
The 1990 base-year emissions estimates in the database range
from 55 to 91 megatons of sulfur (MtS), a seemingly large
difference that can be explained partially by the different extent
of coverage of SOj emissions in different models and scenario
studies, in addition to uncertainties in emissions factors.
Typically, lower range emissions derive from models that report
only (the dominant) energy sector emissions, higher ranges also
include other anthropogenic sources such as SO2 emissions
from métallurgie processes. Differences in 1990 base-year
values across scenario studies and a review of available SO2
emissions inventories are discussed in more detail in Chapter 3.
Indexed to a common 1990 basis, future SO2 emissions trends
reveal a number of remarkable characteristics. Fhst, contrary to
A detailed review of long-term global and regional SO2
emissions scenarios is given in Grübler (1998) and is
summarized in Chapter 3. The most important new finding
from the scenario literature is recognition of the significant
impacts of continued unabated high SO2 emissions on human
health, food production, and ecosystems. As a resuh, scenarios
published since 1995 all assume various degrees of SO2
emissions control and interventions to be implemented in the
future, and are thus substantially lower than previous
projections, including the IS92 series. In most of these
scenarios, such low levels of SO^ emissions are not simply the
result of direct SOj emissions control measures, such as flue
gas desulfurization. They also result from other interventions
in which SO2 emissions reduction is more a secondary benefit
than a primary goal (e.g., structural changes for various reasons
other than SO2 control).
2.4.4. Population Projection Ranges
Population is one of the fundamental driving forces of future
emissions. Most models used to formulate population
projections for the emissions scenarios are taken from the
literature and are exogenous inputs. Today three main research
groups project global population - United Nations (UN, 1998,
1999), World Bank (Bos and Vu, 1994), and IIASA (Lutz et al,
1997). (For more details see the discussion in Chapter 3.) Most
of the "central" population projections lead to a doubling of
global population by 2100 (to about 10 billion people
compared to 5.3 billion in 1990). In recent years the central
population projections for the year 2100 have declined
somewhat, but are still in line with a doubling by 2100. For
example, the latest UN (1998) medium-low and medium-high
projections indicate a range of between 7.2 and 14.6 bilUon
people by 2100, with the medium scenario at 10.4 billion. The
IIASA central estimate for 2100 is also 10.4 bilUon, with a
95% probability that world population would exceed six and be
lower than 17 bilUon (Lutz et al, 1997).
While all scenarios require some kind of population
assumptions, relatively few are reported explicitly in the SRES
database. Figure 2-4 illustrates global population projections in
the database. Of the 416 scenarios currently documented, only
46 report their underlying population projections. This limited
number indicates that, even though population is an extremely
important driving force for emissions, it is typically either not
reported or not well explored in most models. For the small